Use of glass ceramic plates as cooktops in electric cooking apparatus is becoming increasingly common. Among the advantages of this smooth cooking surface is its pleasing appearance and easy cleanability. However, due to the high thermal impedance of the glass ceramic plate, such cooktops are less efficient thermally than conventional cooking surfaces using sheathed heating elements.
Due to unique electrical and thermal characteristics possessed by materials such as molybdenum disilicide (MoSi.sub.2) and tungsten, heating elements made from these materials are attractive for use with glass ceramic cooktops. The high positive temperature coefficient of resistivity, low thermal mass, and low specific heat characteristic of MoSi.sub.2 and tungsten and the high operating temperature achievable using heating elements made from these materials provide the potential for improved thermal efficiency for cooking apparatus which incorporate a glass ceramic cooktop. However, these same dynamic electrical and thermal characteristics create power control problems which have rendered the use of heating elements made from these materials impractical in electric cooking apparatus.
Conventionally, power control in electric cooking apparatus is achieved using temperature sensitive switches, such as bimetalic infinite heat switches. In operation, the operator adjusts the switch to provide the desired cooking temperature. The switch remains closed until the heating element reaches a predetermined temperature. The switch then opens and remains open until the element temperature drops to a predetermined temperature. The switch continues to cycle ON and OFF in this manner indefinitely. Since conventional sheathed heating elements heat up and cool down relatively slowly, these switching cycles are relatively long, ranging from a few seconds to thirty seconds. In addition, the resistance of a conventional sheathed heating element changes only slightly in going from room temperature to operating temperature. Since the resistance of conventional heating elements is relatively independent of temperature in the temperature range of interest, transient current surges when the switches close are minimal. Thus, conventional power control techniques work satisfactorily.
However, the dynamic characteristics of heating elements made from MoSi.sub.2 or tungsten prevent these heating elements from being controlled using conventional control techniques. Firstly, a MoSi.sub.2 heating element, as described in U.S. Pat. No. 3,912,905, typically varies in resistance from 2-3 ohms at room temperature to 25 ohms at an operating temperature of approximately 1000.degree. C. Thus, assuming energization from a standard 240 volt AC household supply, as the temperature of the heating element changes from room temperature to operating temperature, the load current changes from an initial peak of roughly 110 amps to a steady state current on the order of 8.5 amps RMS. This initial current of 110 amps is obviously greater than can be tolerated in a household appliance except for extremely brief periods. Secondly, the heating element cools extremely rapidly; the first time constant for thermal response of this heating element being in the 600-1000 millisecond range. Since the elements cool rapidly with a concurrent drop in resistance, even a very brief termination of applied power followed by a subsequent application of power results in excessive current draw. A very rapid switching capability therefore is required to avoid frequent excessive current surges; brief ON times limit the duration of excessive current during the heat-up of the element; brief OFF times prevent unacceptable drops in resistance during steady state operation by limiting cooling of the element between ON times. Clearly, the relatively slow mechanical switching of the conventionally employed infinite heat switches cannot provide the rapid switching required to prevent excessive current flow during each application of power. Similarly, conventional electronic controls for use with conventional heating elements have been designed to employ relatively long ON and OFF periods.
A further problem created by the dynamic thermal characteristics of MoSi.sub.2, tungsten, or similar heating elements is a visible light flicker phenomenon. These elements glow almost instantaneously with the application of power thereto. As the time between successive power pulses increases, the glow appears to flicker or oscillate between a higher and lower intensity. This flicker is annoying and is desirably eliminated.